1. Field of the Invention
The invention relates to a semiconductor light emitting device and method of manufacturing the same. More particularly, it relates to a semiconductor light emitting device which provides a light of any emission color produced by additive color mixture in which light emitted from a semiconductor light emitting element is combined with light emitted from the semiconductor light emitting element and wavelength-converted by a fluorescent material, and method of manufacturing the same.
2. Description of the Related Art
A (LED) chip operative to emit a light having a sharp spectral distribution can be employed as a light source to realize an LED that emits a white light. In this case, the light emitted from the LED chip is subjected to additive color mixture with a wavelength-converted light created from a fluorescent material when excited by the light emitted from the LED chip.
For example, if the light emitted from the LED chip is a blue light, a fluorescent material is employed that can wavelength-convert the blue light into its complementary yellow light when excited by the blue light. In this case, the blue light emitted from the LED chip is subjected to additive color mixture to yield a white light, with the wavelength-converted yellow light created from the fluorescent material when excited by the blue light emitted from the LED chip.
Similarly, when the light emitted from the LED chip is blue light, two types of fluorescent materials may be employed in combination to wavelength-convert the blue light into respective green and red lights when excited by the blue light. In this case, the blue light emitted from the LED chip is subjected to additive color mixture to yield a white light, with wavelength-converted green and red lights created from the two types of fluorescent materials when excited by the blue light emitted from the LED chip.
If the light emitted from the LED chip is an ultraviolet light, three types of fluorescent materials may be employed in combination to wavelength-convert the ultraviolet light into blue, green and red lights respectively when excited by the ultraviolet light. In this case, the ultraviolet light emitted from the LED chip excites the three types of fluorescent materials to create the wavelength-converted blue, green and red lights, which are subjected to additive color mixture with each other to yield a white light.
Further, the emission color of light emitted from the LED chip can be combined appropriately with any fluorescent material serving as a wavelength converter to create various toned colors other than white light.
FIG. 4 shows an example of an LED in which fluorescent material is excited by light emitted from the light source for wavelength conversion to provide a toned light different from the light emitted from the light source, as described above. A light emitting element 51 is disposed on the bottom in a cup 52 and electrically connected thereto through a bonding wire 56. A fluorescent material 53 serving as a wavelength converter is dispersed in a resin 54, which is filled in the cup 52. A lid is provided to close the top of a casing 55, which is then turned upside down for thermal setting of the resin 54. Thus, the fluorescent material 53 having a larger specific gravity than that of the resin sinks and collects in the upper portion of the cup 52. As a result, the fluorescent material 53 is more densely distributed in the upper portion than the lower portion of the cup 52 in a finished LED (see JP-A 2002-151743, for example).
In another example shown in FIG. 5, an LED chip 61 is disposed on the bottom in a cup 63 and electrically connected thereto through a bonding wire 62. A first light-transmissive resin 64 is filled in the cup 63 by about 60-70% of the cap volume and heated. A second light-transmissive resin 66 is further injected onto the first resin by about 50-60% of the cap volume. The second resin contains a fluorescent material 65 serving as a wavelength converter dispersed in a light transmissive resin. This device is turned upside down for thermal setting. Consequently, the second light-transmissive resin 66 expands to project at the outer rim of the cup 63, and the fluorescent material 65 dispersed in the second light-transmissive resin 66 sinks and collects in the upper portion that is expanded outwards in a projected convex surface. As a result, the fluorescent material 65 is more densely distributed in the vicinity of the upper surface that is formed like a convex lens in the finished LED (see JP-A 2003-234511, for example).
In the above-described conventional LEDs, the former requires thermal setting of the resin that is filled in the cup to be performed in a state in which a lid is placed on the top of the casing and the casing is turned upside down. Therefore, the top of the casing must be brought entirely into intimate contact with the lid without leaving any gap therebetween. If even a little gap is present, the resin flows out through the gap, causing a failed product.
In particular, if a number of cups are formed in a large casing for mass production in batch, it is difficult to ensure an extremely high surface accuracy that would bring the cup tops into intimate contact with the lid over the entire surface. Even if possible, it obviously results in high costs. In addition, heat during thermal setting of the resin can cause deformations such as expansions and deflections in both the casing and the lid. Such deformations increasingly prevent the intimate contact between both members and inevitably lead to poor production yield.
Further, in order to increase the amount of light emission, the light emitting element can be upsized to allow a large current to flow therethrough. In practice, however, the package has a limit in size. Therefore, the cup that is employed to hold the light emitting element therein is also limited in size. As a result, the light emitting element has a larger proportion in the inner volume of the cup than a conventional LED of the same type. In other words, a spatial volume of the cup, obtained by subtracting the volume of the light emitting element from the inner volume of the cup, has a decreased ratio to the inner volume of the cup.
As a result, the distance between the side of the light emitting element and the inner circumferential surface of the cup approaches the distance between the upper surface of the light emitting element and the top of the fluorescent material-dispersed resin that is filled in the cup. In this case, a larger amount of resin is present between the side of the light emitting element and the inner circumferential surface of the cup as compared to the amount of resin present between the upper surface of the light emitting element and the top of the resin. This relation is similarly found between the amounts of the fluorescent material dispersed in the resin.
Again, a lid can be placed on the casing, which is then turned upside down for thermal setting of the resin. As a result, the fluorescent material having a larger specific gravity than that of the resin sinks and collects in the upper portion of the cup to form the LED. The fluorescent material is more densely distributed in the upper portion than the lower portion of the cup. In this case, a larger amount of the fluorescent material in the resin is present between the side of the light emitting element and the inner circumferential surface of the cup as compared to the amount of fluorescent material present between the upper surface of the light emitting element and the top of the resin. Therefore, when the device is turned upside down for thermal setting of the resin, a larger amount of fluorescent material is precipitated around the light emitting element as compared to the amount of the fluorescent material precipitated above the light emitting element. This makes it difficult to form a uniform layer of fluorescent material.
As a result, the light emitted from the light emitting element to the layer of fluorescent material excites the fluorescent material to varying degrees depending on location. Therefore, a problem arises because the light source has color variations. In an LED configured to emit a white light, color variations are strictly regulated in practice because variation creates a high possibility of degrading the light yield.
In the latter, on the other hand, the first light-transmissive resin climbs up to the outer rim of the cup due to surface tension. In this condition, the second, fluorescent material-dispersed, light-transmissive resin forms a convex-lens-like projection on the first resin to provide a layer of high-density fluorescent material near the surface of the convex-lens-like projection. In this case, the amount of fluorescent material is less at the ends as compared to at the convex projection of the second light-transmissive resin. In addition, the second light-transmissive resin may not sufficiently reach the “climbed” portion of the first light-transmissive resin, and it is possible that no layer of fluorescent material is formed therein.
Originally, an LED was desired in which additive color mixture of the light emitted from the light emitting element with the wavelength-converted light produces a white light with no color variation in almost all directions. A light emitted from the light emitting element and directly radiated from the LED without passing through the layer of fluorescent material may be present within a certain region. In that region, only the light emitted from the light emitting element (not the light produced through additive color mixture) is radiated as it is.
In this case, if the light emitted from the light emitting element is a blue light having a peak emission wavelength of about 450-470 nm, the light emitted from the light emitting element, guided through a region with the layer of fluorescent material formed therein, and radiated from the LED becomes a white light (W). In contrast, the light emitted from the light emitting element, guided through a region with no layer of fluorescent material formed therein, and radiated from the LED becomes a blue light (B). Therefore, such an LED radiates a light with color variations in white and blue and is not an excellent white LED product.
The light emitted from the light emitting element may have a peak emission wavelength within a short wavelength range of about 400 nm or less. If such an ultraviolet light is directly radiated from the LED and enters human eyes, it may cause some ill effect, which is not preferred.
The present invention has been made in consideration of the above and various other problems and issues, and accordingly can provide a semiconductor light emitting device, which may serve as a light source that has less variation in color and brightness and can reduce radiation of light that may possibly be harmful to humans, and a method of manufacturing the same.